Among steam turbines, there is widely used a steam turbine of an axial flow type in which a plurality of turbine stages each composed of a stationary blade cascade and a rotor blade cascade are arranged in a turbine rotor axial direction in which steam flows. A compact structure is required of such a steam turbine in view of improving space efficiency.
The rotor blade cascades in the steam turbine each include a plurality of rotor blades which are implanted in a circumferential direction of a turbine rotor. On the other hand, as for the stationary blade cascades, some has a plurality of stationary blades which are arranged in the circumferential direction between a diaphragm outer ring and a diaphragm inner ring, and some other has a plurality of stationary blades which are arranged in a circumferential direction on an inner circumference of a casing.
FIG. 22 is a view showing a meridian cross section of a conventional steam turbine including stationary blade cascades 310 between a diaphragm outer ring 312 and a diaphragm inner ring 314. In FIG. 22, a single turbine stage composed of the stationary blade cascade 310 and a rotor blade cascade 320 is shown.
The stationary blade cascade 310 is formed between the diaphragm outer ring 312 which has a groove 311 opening toward an inside diameter side and continuing in a circumferential direction of the diaphragm outer ring 312 and the diaphragm inner ring 314 which has a groove 313 opening toward an outside diameter side and continuing in a circumferential direction of the diaphragm inner ring 314. Stationary blades 315 each include, on its outer circumference side, an implantation portion 316 for diaphragm outer ring, and the implantation portions 316 for diaphragm outer ring are fitted in the groove 311.
The stationary blades 315 each include, on its inner circumference side, an implantation portion 317 for diaphragm inner ring, and the implantation portions 317 for diaphragm inner ring are fitted in the groove 313. That is, the stationary blades 315 are supported on the diaphragm outer ring 312 and the diaphragm inner ring 314 not by welding but by fitting. Further, on an outer circumference of the diaphragm outer ring 312, a casing 330 is provided to prevent high-temperature, high-pressure steam from leaking outside.
FIG. 23 is a view showing a meridian cross section of a conventional steam turbine including stationary blade cascades 355 each having stationary blades arranged in a circumferential direction on an inner circumference of a casing 350. As shown in FIG. 23, fitting grooves 351 are formed all along the circumferential direction in the inner circumference of the casing 350. Fitting portions 353 of stationary blades 352 are fitted in the fitting grooves 351 to be fixed to the casing 350, whereby the stationary blade cascades 355 are formed. Further, pressure pins 354 press the stationary blades 352 radially inward in order to firmly fix the stationary blades 352 to the casing 350.
In the conventional steam turbine including the stationary blade cascades 310 between the diaphragm outer ring 312 and the diaphragm inner ring 314, a clearance δr for allowing thermal expansion is provided between the casing 330 and the diaphragm outer ring 312 as shown in FIG. 22. That is, an inside diameter of the casing 330 is decided by an outside diameter of the stationary blades 315, a radial thickness of the diaphragm outer ring 312, the clearance δr, and so on.
Here, the outside diameter of the stationary blades 315 is a dimension set for optimizing performance depending on a stem flow rate and a steam condition, and the clearance δr is set in order to allow the thermal expansion, and their great changes are not allowed.
Further, for example, between the groove 311 and the implantation portions 316 for diaphragm outer ring, a slight gap is formed all along the circumferential direction. Therefore, on horizontal end surfaces (horizontal joint surfaces) of the diaphragm outer ring 312 having a two-divided structure of an upper half and a lower half, fastening bolts for fastening the upper half and the lower half and pins, keys, and the like for positioning need to be provided in order to prevent the leakage of steam. However, reducing the radial thickness of the diaphragm outer ring 312 necessitates the downsizing of the fastening bolts, pins, keys, and so on. This results in insufficient fastening force and positioning to cause a problem that the steam easily leaks at the horizontal end surfaces.
As described above, in the conventional steam turbine including the stationary blade cascades between the diaphragm outer ring and the diaphragm inner ring, it has been difficult to realize the downsizing.
In the conventional steam turbine including the stationary blade cascades 355 each having the stationary blades arranged in the circumferential directionon the inner circumference of the casing 350, the stationary blade cascades 355 expand radially inward and a turbine rotor 356 and the casing 350 expand radially outward at the time of the thermal expansion, as shown by the arrows in FIG. 23. At this time, the expansion of the casing 350 is small but the expansion of the stationary blade cascades 355 and the turbine rotor 356 is large. Accordingly, a gap between the stationary blade cascades 355 and the turbine rotor 356 becomes small, which has a risk that they come into contact with each other to cause a significant accident.